In gravure printing a cylindrical form is used to print what is desired and this is done by a photoengraving process during which tiny cells are formed that receive the ink subsequently transferred onto paper.
The surface on which these cells are engraved by various methods--traditional, halftone, conversion, electronic etc.,--is generally of copper. Said surface is obtained by galvanic deposition on steel cylinders and, after photoengraving, is given a chromium protective process.
For each fresh set of pictures the cylinders are processed by specialized engravers and are then sent to the user who mounts them onto machines for gravure printing.
Bearing in mind the considerable size and weight of each cylinder as well as the great delicacy of the surfaces, already engraved or prepared for it, there are clearly problems of transport and storage with related costs and capital tied up for the purpose.
To overcome these drawbacks and lower costs, metal cylindrical sleeves, usually nickel, are used, to slide on and off the cylinders.
A coating of copper, like that deposited on traditional cylinders, is deposited on the surfaces of these sleeves for good engraving results. There are a various methods for assisting the work of removing sleeves from these cylinders. The best known comprises having small holes made in the surface of the cylinder to permit creation of a cushion of compressed air between the surface of the cylinder and that of the sleeve.
Considering the great difference between the weight of a sleeve, 2-3 kg., and that of the cylinder, 100-200 kg., it is clearly advantageous to have a removable sleeve rather than a conventional type of cylinder. Running costs for corrections, balancing, maintenance, staff and transport are all reduced.
The quality of print is higher, while adjusting movements are avoided as well as waste of costly materials.
In the present situation of the market, however, requiring a greater variety of packing materials, which in turn means shorter printing runs and the ever present need to lower costs to increase competitivity, the fact that sleeves still have to be eliminated after use remains a serious loss and has a negative effect on the market.
A process has therefore been devised whereby a copper film is formed on the nickel sleeve and a separator product interposed between the two so that the film can easily be slid off when a new engraving is required. A fresh film for a fresh engraving is then deposited by electroplating on the nickel sleeve.
The separator product generally consists of a silver-based solution or of passivating substances, bichromates or some others. According to a preferred method a copper film and the separator product are applied to the nickel sleeve followed by formation of the copper film to be photoengraved.
To detach the copper film from the sleeve, for a fresh photoengraving process, use has to be made of mechanical means.
After this a new film is deposited following application of the separator. This process offers considerable advantages compared with direct deposition on the cylinder of the film to be photoengraved.
Even so, however, sleeve costs are high because of the high cost of nickel and because of the time needed to form the different layers: the copper film, the separator product and the copper film for engraving.